click on file
Click save
We've got the name "subscribe'
Now we're going to select PNG transparent full quality, 3KB
Click OK
Save to your desktop
That's it!
Choose the file you just created
Click subscribe.png
Click "Choose"
Click save
Click save again
It says "showing on all your uploads"
Choose "entire video" from the drop down
Update
Check that the subscribe button goes to the channel. If nobody has subscribed you'll see the subscribe button at the bottom of your YouTube Channel banner.
That's it!

The subscribe button will appear in the top right corner of all your uploaded videos across all devices.